1. Field of the Invention
The present invention relates generally to a demodulation and modulation circuit and demodulation and modulation method. More particularly, the invention relates to a demodulation and modulation circuit and demodulation and modulation method of a digital transmission signal to be used in a cellular phone terminal.
2. Description of the Related Art
Conventionally, sampling in an analog-to-digital (A/D) converter in a demodulation circuit causes phase shift due to a frequency offset between transmission and reception and thus cannot be performed constantly at optimal sampling timing.
Therefore, at certain sampling timing, it is possible to significantly degrade reception characteristics for occurrence of error in symbol judgment due to sampling of the reception signal close to zero crossing (boundary point in transition where symbol is changed from positive to negative or negative to positive).
The following equation shows adverse influence of the frequency offset for demodulation. It is assumed that a modulated wave is expressed by:s(t)=A(t) cos [2πfct+φ(t)]
Here, A(t) is assumed to be +1 or −1, and a carrier wave component cos [2πfct] of the modulated wave set forth above is a reference signal pi(t), an orthogonal demodulator output I component is expressed by:
                              I          ⁡                      (            t            )                          =                ⁢                              s            ⁡                          (              t              )                                ⨯                      pi            ⁡                          (              t              )                                                              =                ⁢                              A            ⁡                          (              t              )                                ⁢                                    cos              ⁡                              [                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    fct                                    +                                      ϕ                    ⁡                                          (                      t                      )                                                                      ]                                      ⨯                          cos              ⁡                              [                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  fct                                ]                                                                            =                ⁢                              (                                          A                ⁡                                  (                  t                  )                                            /              2                        )                    ⨯                      [                                          cos                ⁡                                  (                                                            4                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      fct                                        +                                          ϕ                      ⁡                                              (                        t                        )                                                                              )                                            +                              cos                ⁢                                                                  ⁢                                  ϕ                  ⁡                                      (                    t                    )                                                                        ]                              
By cutting off cos (4πfct+φ(t)) as the second term on right side by LPF, the I component can be expressed by:I(t)=(A(t)/2)×cos φ(t)to obtain phase information of I component of a PSK modulated wave.
Similarly, a carrier wave component −sin [2πfct] which is obtained by shifting the phase for π/2 ahead of the modulated wave is a reference signal pq(t), an orthogonal demodulator output Q component is expressed by:Q(t)=A(t) cos [2πfct+φ(t)]×(−sin [2πfct])=A(t)/2)×cos φ(t)
However, since the frequency offset Δθ(t) is caused between transmission and reception in the practical circuit, respective reference signals can be expressed by:pi(t)=cos [2πfct+Δθ(t)]pq(t)=−sin [2πfct+Δθ(t)]The orthogonal demodulation output is expressed by multiplying the foregoing reference signal and the modulated wave and cutting off a high frequency component by the LPF:
                              I          ⁡                      (            t            )                          =                ⁢                              A            ⁡                          (              t              )                                ⁢                                    cos              ⁡                              [                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    fct                                    +                                      ϕ                    ⁡                                          (                      t                      )                                                                      ]                                      ⨯                          cos              ⁡                              [                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    fct                                    +                                      Δ                    ⁢                                                                                  ⁢                                          θ                      ⁡                                              (                        t                        )                                                                                            ]                                                                            =                ⁢                              (                                          A                ⁡                                  (                  t                  )                                            /              2                        )                    ⁢                      cos            ⁡                          (                                                ϕ                  ⁡                                      (                    t                    )                                                  -                                  Δ                  ⁢                                                                          ⁢                                      θ                    ⁡                                          (                      t                      )                                                                                  )                                                                        Q          ⁡                      (            t            )                          =                ⁢                              A            ⁡                          (              t              )                                ⁢                                    cos              ⁡                              [                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    fct                                    +                                      ϕ                    ⁡                                          (                      t                      )                                                                      ]                                      ⨯                          -                              sin                ⁡                                  [                                                            2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      fct                                        +                                          Δ                      ⁢                                                                                          ⁢                                              θ                        ⁡                                                  (                          t                          )                                                                                                      ]                                                                                            =                ⁢                                            (                                                A                  ⁡                                      (                    t                    )                                                  /                2                            )                        ⨯                          cos              ⁡                              (                                  ϕ                  ⁡                                      (                    t                    )                                                  )                                              -                      Δ            ⁢                                                  ⁢                          ϕ              ⁡                              (                t                )                                                        Thus, adverse influence of the frequency offset appears on the orthogonal demodulation output. By this, the phase of an input signal of the A/D converter is shifted to cause offset from a desired sampling timing.
Examples of the prior art of this kind have been disclosed in (1) Japanese Unexamined Patent Publication No. Heisei 8-223132, (2) Japanese Unexamined Patent Publication No. Heisei 10-260653 and (3) Japanese Patent No. 2570126 (hereinafter referred to as prior art 1 to 3.
The prior art 1 is designed to insert a pilot signal to the transmission signal, derive a frequency offset Δk and a synchronization offset δ of the sampling timing on the basis of the transmission frequency k of the pilot signal and the reception frequency k′, and control the sampling period and a transmission frequency of a frequency converter so as to reduce the foregoing offsets to zero.
The prior art 2 is designed for controlling a delay amount of the sampling clock and for controlling the phase of the sampling clock of the input video signal S1 to the phase adapted for the input video signal S1.
The prior art 3 is designed for extracting a clock signal component from a demodulation base band signal and outputting a signal synchronized with the clock signal component as a sampling clock.
As set forth above, in the foregoing prior arts 1 to 3, the ploblem that the phase of the input signal of the A/D converter is shifted to cause offset from the desired sampling timing is solved by controlling the sampling frequency.
However, when the sampling frequency in the A/D converter is increased in order to reduce error in symbol judgment, increasing of power consumption is caused in proportion to increasing of the frequency.
Increasing of power consumption causes significant problem in the equipment desired to be compact and to be used for a long period, such as a communication terminal, e.g. current cellular phone terminal.